A Promise signature allows for the compilation of function bodies that return undefined

The compiler error that I expected to see when using this function does not actually occur. The function body is capable of returning undefined, yet the type signature does not mention this possibility.

  async function chat(_: at.ChatLine): Promise<Array<tot.PlayStateAction>> {
    return lastMover?.chat(_);
  }

tsconfig.json

{
  "compilerOptions": {
    "target": "es2019",
    "module": "commonjs",
    "outDir": "./lib",
    "sourceMap": true,
    "declaration": true,
    "strict": true,
    "incremental": true,
    "esModuleInterop": true
  },
  "include": ["src"],
  "exclude": ["**/node_modules", "**/.*/"]
}

Answer №1

To resolve the issue, make sure to activate strictNullChecks in your tsconfig.json file. It is also necessary for either the declared type of lastMover or the inferred type to include null and/or undefined. You can replicate the problem with the following code snippet:

let lastMover: LastMover | undefined | null;

If not configured correctly, you will encounter this error message:

Type 'XXX[] | undefined' cannot be assigned to type 'XXX[]'.
Type 'undefined' cannot be assigned to type 'XXX[]'.

(replace XXX with tot.PlayStateAction based on your scenario)

Note: Despite the documentation stating that strict automatically enables strictNullChecks, this does not apply in the TS Playground (click here - deselect strictNullChecks to see the absence of errors).

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